The overall objective of this Program Project is to identify and refine strategies for the prevention and management of dengue. Dengue continues to be an expanding public health problem, disproportionally affecting resource-poor countries in tropical and subtropical regions. Although morbidity from clinically mild infections is still considerable, the principal challenge presented by dengue virus is its ability to cause dengue hemorrhagic fever (DHF), a potentially fatal plasma leakage syndrome. There is no specific therapy or vaccine available against dengue, and development of treatments or vaccines has been problematic because of the evidence that DHF is immunologically mediated. In this application, an experienced interdisciplinary team of university, military, and industry investigators in the US and Thailand proposes to conduct coordinated studies to a) advance understanding of DHF epidemiology, pathophysiology, and immunopathogenesis and b) use this knowledge to identify and validate approaches to management and prevention of dengue disease. Project 1 (Clinical Studies) will involve a prospective study in Bangkok of symptomatic children hospitalized with acute dengue illness, to define the pathophysiology of severe dengue and improve the evaluation and triage of cases, and a prospective population-based study of dengue transmission and disease in Kamphaeng Phet province, to take place concurrently with a phase lib vaccine efficacy trial, to define optimal methods for evaluation of vaccine efficacy and identify correlates of protective immunity. Project 2 (Molecular Immunopathogenesis) will involve detailed laboratory studies of innate and adaptive immune responses to dengue in vitro and in vivo to define the immunologic mechanisms underlying protection and disease pathogenesis. A Clinical Laboratory Core and an Administrative Core will support both Projects. Close interactions and sharing of data between these Projects and Cores will ensure the maximum yield from these research studies, with broad basic science as well as clinical and public health implications. This research Program involves studies in the U.S. and Thailand to understand how the mosquito-borne dengue virus causes disease in people and how it can best be treated or prevented by vaccination. PROJECT 1: Clinical Studies on the Mechanisms of DF/DHF (Green, Sharone) PROJECT 1 DESCRIPTION (provided by applicant): The overall goal of Project 1 (Clinical Studies) is to identity the clinical, immunologic, virologic and epidemiologic factors that have the strongest influence on determining the ultimate clinical manifestations of dengue virus infections in Thai children. This Project will involve two clinical studies. The first will be a prospective study in Bangkok of symptomatic children hospitalized with acute dengue illness to define the pathophysiology of severe dengue. The findings of this study should be useful to guide the evaluation and triage of febrile patients with suspected dengue as well as to improve the assessment of capillary leakage in DHF. We plan to utilize noninvasive technologies such as echocardiogram, ultrasonography, and measures of heart rate variability. Novel near infrared spectroscopy will be used to noninvasively determine hematocrit, muscle pH and muscle pO2. We also will perform flow cytometric assays to identify and characterize circulating endothelial cells as well as virus-specific T cells during acute infection. Our second study will involve a Phase lib vaccine efficacy trial that will take place in Kamphaeng Phet, Thailand. Research studies conducted during this unique opportunity will analyze the effects of dengue vaccination on viral transmission and define immunologic correlates of protection or possible sensitization to more severe illness. These research studies would not be performed by the vaccine manufacturer and will be crucial for the design and implementation of future clinical field trials of dengue vaccines. These clinical studies will be supported by Cores A and B and will provide valuable specimens (serum, peripheral blood mononuclear cells and virus isolates) for studies to be performed in Project 2. This Project includes two clinical studies in Thailand: (1) in hospitalized children, how the mosquito-borne dengue virus causes clinical disease and (2) in a field trial of a candidate dengue vaccine, to study how vaccines may protect against dengue virus infection.